"A new analysis of licensing data shows that not only is use of the GPL and other copyleft licenses continuing to decline, but the rate of disuse is actually accelerating." This shouldn't be surprising. The GPL is complex, and I honestly don't blame both individuals and companies opting for simpler, more straightforward licenses like BSD or MIT-like licenses.

The Kernel is licensed under GPL V2 Only. Not GPL V2 or any later version.
He's not the only contributor to the kernel, as such he would need the explicit approval of everyone to ever contribute code that is present in the kernel in order to re-license it as GPL V3. That is simply not practical.

Linux has changed license in the past. It would be very, very hard, but not impossible. For example, Linus previously stated that, if OpenSolaris were to be released under GPLv3, he might consider changing Linux's license to GPLv3. Of course, that didn't happen and it was probably the only change of a license change ever happening.